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United States Patent |
6,175,118
|
Takayama
,   et al.
|
January 16, 2001
|
Gamma camera
Abstract
According to the present invention, sub-windows using a TEW technique are
centered to energies corresponding to 1/n and 1/m of maximal photon number
in a standard energy spectrum without any scattering component. It is thus
possible to improve a count coefficiency, while broadening a main window
to a maximal possible extent, without underestimating the scattering
component and crosstalk component.
Inventors:
|
Takayama; Takuzo (Otawara, JP);
Ichihara; Takashi (Otawara, JP);
Motomura; Nobutoku (Nasu-gun, JP)
|
Assignee:
|
Kabushiki Kaisha Toshiba (Kawasaki, JP)
|
Appl. No.:
|
119624 |
Filed:
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July 21, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
250/369; 250/363.07; 250/371 |
Intern'l Class: |
G01T 001/161 |
Field of Search: |
250/363.07,363.09,369,371,252.1,362
|
References Cited
U.S. Patent Documents
5371672 | Dec., 1994 | Motomura et al.
| |
5633500 | May., 1997 | Morgan et al. | 250/363.
|
Other References
T. Takayama, et al. "Determination of Energy Window Width and Position For
Scintigraphic Imaging Using Differnet Energy Resolution Detection With The
Triple Energy Window (TEW) Scatter Compensation Method", IEEE 1998,
Conference Record
|
Primary Examiner: Hannaher; Constantine
Assistant Examiner: Gagliardi; Albert
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. A gamma camera comprising:
a camera head for detecting photons radiated from RIs administered to a
human subject or from an external ray source;
means for counting, out of the detected photons, those photons whose energy
enters within a main window, the counted photon number being given as a
first value;
means for counting, out of the detected photons, those photons whose energy
enters within a first sub-window arranged to a high energy side relative
to the main window, the counted photon number being given as a second
value, the first sub-window being centered to an energy corresponding to
the number of photons of MAX/n in a standard energy spectrum where
n represents a rational number exceeding one,
the standard energy spectrum represents an energy distribution of the
photon number relating to direct rays from the RIs without scattering
rays, and
MAX represents a maximal photon number of the standard energy spectrum;
means for counting, out of the detected photons, those photons whose energy
enters within a second sub-window arranged to a low energy side relative
to the main window, the counted photon number being given as a third
value, the second sub-window being centered to an energy corresponding to
the photon number of MAX/m in the standard energy spectrum where
m represents a rational number exceeding one, and
means for correcting the first value on the basis of the second and third
values.
2. A gamma camera apparatus according to claim 1, wherein the n and m are
selected from a 100.+-.10% range.
3. A gamma camera apparatus according to claim 1, wherein the n is set to
be equal in value to the m.
4. A gamma camera apparatus according to claim 1, wherein the standard
energy spectrum is approximated to a Gaussian distribution specified from
a photoelectric peak inherent in the RIs and an energy resolution inherent
in the camera head.
5. A gamma camera apparatus according to claim 1, further comprising means
for reconstructing a concentration distribution of the RIs or an
absorption coefficient map of the human subject.
6. A gamma camera apparatus according to claim 1, wherein the first
sub-window is set to a width of a % of a photoelectric peak inherent in
the RIs, the second sub-window is set to a width of b % of the
photoelectric peak, and the main window is set to be present between the
set first sub-window and the set second sub-window.
7. A gamma camera comprising:
a camera head for detecting photons radiated from RIs administered to a
human subject or from an external ray source;
means for counting, out of the detected photons, those photons whose energy
enters within a main window, the counted photon number being given as a
first value;
means for counting, out of the detected photons, those photons at each
incident position whose energy enters within a first sub-window arranged
to a high energy side relative to the main window, the counted photon
number being given as a second value;
means for counting, out of the detected photons, those photons at each
incident position whose energy enters within a second sub-window arranged
to a low energy side relative to the main window, the center of the second
sub-window being arranged between a photoelectric peak inherent in the RIs
and a peak of a scattering ray associated with the photoelectric peak, the
counted photon number being given as a third value; and
means for correcting the first value on the basis of the second and third
values.
8. A gamma camera apparatus according to claim 7, wherein, when a nuclide
of the administered RIs or the external ray source is Ti-201, the center
of the second sub-window is arranged in a 51 to 56 KeV range.
9. A gamma camera apparatus according to claim 7, wherein the second
sub-window is set to a width of a % of a photoelectric peak inherent in
the RIs, the main window is set to be width of b % of the photoelectric
peak arranged to the set second sub-window and the first sub-window is set
to be a width of c % of the photoelectric peak arranged to the set main
window.
10. A gamma camera comprising:
a camera head for detecting photons radiated from RIs administered to a
human subject or from an external ray source;
means for counting, out of the detected photons, those photons whose energy
enters within a main window, the photon number being given as a first
value,
means for counting, out of the detected photons, those photons whose energy
enters within a first sub-window arranged to a high energy side relative
to the main window, the photon number being given as a second value;
means for counting, out of the detected photons, those photons whose energy
enters within a second sub-window arranged to a low energy side relative
to the main window, the photon number being given as a third value;
means for correcting the first value on the basis of the second and third
values; and
means for adjusting the main window, first sub-window and second sub-window
in accordance with a nuclide of the RIs or external ray source, the first
sub-window being centered to an energy corresponding to the photon number
of MAX/n in a standard energy spectrum, and the second sub-window being
centered to an energy corresponding to the photon number of MAX/m in the
standard energy spectrum, where
the n and m represent a rational number exceeding one;
the standard energy spectrum represents an energy distribution of the
photon number relating to direct rays from the RIs; and
the MAX represents a maximum photon number in the standard energy spectrum.
11. A data processing method for correcting data acquired by a gamma
camera, out of photons radiated from RIs administered to a human subject
or from an external ray source, the number of photons whose energy enters
a main window being corrected on the basis of the number of photons whose
energy enters a first sub-window arranged to a high energy side relative
to the main window and the number of photons whose energy enters the
second sub-window arranged to a low energy side relative to the main
window, the correcting method comprising the steps of:
centering the first sub-window to an energy corresponding to the number of
photons of MAX/n in a standard energy spectrum, and
centering the second window to an energy corresponding to the number of
photons of MAX/m in a standard energy spectrum,
the n and m represent a rational number exceeding a unity;
the standard energy spectrum represents an energy distribution of the
photon number relating to direct rays from the RIs without any scattering
ray; and
the MAX represents a maximum photon number of the standard energy spectrum.
12. A data processing method for correcting data acquired by a gamma
camera, out of photons radiated from RIs administered to a human being or
from an external ray source, the number of photons whose energy enters a
main window being corrected on the basis of the number of photons whose
energy enters a first sub-window arranged to a high energy side relative
to the main window and the number of photons whose energy enters a second
sub-window arranged to a low energy side relative to the main window, the
correcting method comprising arranging a center of the second sub-window
in a range between a photoelectric peak inherent in the RIs and a peak of
a scattering ray associated with the photoelectric peak.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a gamma camera for detecting a gamma ray
radiated from radio-isotope (hereinafter referred to as RIs) administered
to a human subject and imaging an internal concentration distribution of
the RIs.
The gamma camera is classified into a type of imaging using a single photon
nuclide emitting one photon at a decay of the RIs and a type of imaging
using a positron nuclide emitting a pair of photons in opposite directions
at a quenching of a positron. Recently, the imaging method has been
diversified to cover the following methods.
(Static Imaging)
The imaging method is to obtain RIs distribution (plane image) by detecting
photons, in a predetermined time period, with a single camera head fixed
to a human subject and counting them.
(SPECT Imaging)
This imaging method comprises rotating one camera head around the human
subject, while repeating the detection and counting of photons, and
reconstructing RIs distribution (cross-sectional image), as in a CT
scanning, on the basis of a count value obtained.
(Two Camera Heads-Opposed SPECT Imaging)
This imaging method comprises rotating two camera heads oppositely arranged
with a human subject located therebetween while maintaining this
positional relation, detecting/counting photons during this time period,
and reconstructing a cross-sectional image, as in a CT scanning, on the
basis of a count value obtained.
(Two Camera Head 90.degree. Displaced SPECT Imaging)
This imaging method comprises rotating two camera head 90.degree. displaced
around a rotation axis while maintaining this positional relation,
repeating the detection and counting of photons during this time period,
and reconstructing a cross-sectional image, as in a CT scanning, on the
basis of a count value obtained.
(Three Camera Head SPECT Imaging)
This imaging method comprises arranging three camera heads in a triangular
array, rotating these cameras around a human subject while rotating these
cameras while maintaining this positional relation, repeating the
detection and counting of photons during this time period, and
reconstructing a cross-sectional image, as in a CT scanning, on the basis
of a count value obtained.
In these various imaging methods, in order to improve the image quality and
quantitative property, various corrections are required, such as an energy
correction, a linear correction for correcting a deformation in a marginal
edge of a visual field, a uniformity correction for uniforming a variation
in sensitivity of a photomultiplier, a scattering ray correction for
eliminating scattering components, a crosstalk correction for correcting a
crosstalk between two kinds of RIs differing in their photoelectric peaks,
an absorption correction for correcting a count error resulting from the
non-uniform coefficient of a living body, and so on.
A triple energy window (TEW) method is an excellent correction technique
effective to not only the scattering correction but also a crosstalk
correction. The TEW method requires three energy windows. The three energy
window comprises, as shown in FIG. 1, one main window and two sub-windows.
The main window has its center arranged at a photoelectric peak (Epeak) of
a target nuclide. The two sub-windows are arranged one at each near side
of the main window.
A scattering component (cross-hatched section) mixing into the main window
is estimated by a trapezium approximation calculation from a calculated
value of the two sub-windows. The estimated scattering component is
subtracted from the calculated value of the main window. From the RIs it
is possible to obtain the number of primary photons involved.
The greater the width of the main window, an image can be formed with many
more photons. If the width of the main window is too greater, an amount of
scattering lines mixed becomes greater, thus resulting in a lowering in an
S/N ratio. It has been conventional practice to set the width of the main
window to be 20% of the photoelectric peak Epeak corresponding to a
maximum frequency or that of the sub-window to be 7% of the photoelectric
peak Epeak, not depending upon the nuclide involved.
In the conventional method by which the window width is uniformly set in
this way, there is a tendency that the width becomes too narrow at a
relatively low photoelectric peak, for example, TI-201. This has been thus
far indicated, but the setting method for optimizing these windows for
respective nuclides has not yet currently established.
In order to make the above-mentioned absorption correction, it is necessary
that the spatial distribution of an absorption coefficient on the human
subject be measured with the use of an external ray source of a spatially
uniform photon radiation frequency. The spatial distribution of the
absorption coefficient can be found as follows. That is, the photons
radiated from the external ray source are detected with a camera head
after they have been transmitted through the human subject. The counting
of only the photons whose energies are in the windows is made for
respective incident positions and this is continued for a predetermined
time period. The number of photons emitted for this time period from the
external ray source is known and, being given as "I.sub.0 ", then a
relation below is established:
I.sub.1 =I.sub.0.multidot.e.sup.-.mu..multidot.d
where
I.sub.1 : the number of photons transmitted through the human being, that
is, a count value;
.mu.: the absorption efficient; and
d: the thickness of the human subject.
From this relation it is possible to find the absorption coefficient .mu..
By correcting a count value of the photons from the RIs, administered into
the human subject, on the basis of the absorption coefficient it is
possible to compensate for a count error resulting from a difference of
the absorption coefficient.
In order to correct the scattering ray, the TEW method has been used even
in finding a spatial distribution of the absorption coefficient. Since,
however, the window is not optimized as set out above, there is a strong
tendency that, in this case, the scattering ray is principally
underestimated, thus presenting the problem of the absorption coefficient
being lower than in an actual instance.
BRIEF SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to achieve the
optimization of a main window and sub-windows in the TEW technique
effective to scattering and crosstalk corrections.
According to the present invention, the sub-windows using a TEW technique
are centered to energies corresponding to 1/n and 1/m of maximal photon
numbers in a standard energy spectrum without any scattering component. It
is, thus, possible to improve a counting efficiency, while broadening a
main window to a maximal possible extent, without underestimating
scattering and crosstalk components.
According to the present invention, the low side sub-window using the TEW
technique has its center arranged in a range between a photoelectric peak
inherent in the nuclides of RIs and external ray source and a peak of a
scattering ray associated with this photoelectric peak. It is possible to,
while broadening the main window to a maximal possible extent, improve a
counting efficiency without underestimating any scattering and crosstalk
components.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out
hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention, and together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
FIG. 1 is a view showing windows set by a conventional method;
FIG. 2 is a block diagram showing an arrangement of a major section of a
gamma camera according to a first embodiment of the present invention;
FIG. 3 is an explanatory view for explaining a main window, high side
window and low side window set by a window controller in FIG. 2;
FIG. 4 is a view showing an optimal position of the low side sub-window;
FIG. 5 is a block diagram showing an arrangement of a major section of a
gamma camera according to a second embodiment of the present invention;
and
FIG. 6 is a block diagram showing an arrangement of a major section of a
gamma camera according to a third embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A gamma camera according to preferred embodiments of the present invention
will be explained in more detail below with reference with the drawing. In
this connection, the gamma camera is known, one type being a type of
imaging using a single photon nuclide emitting a single photon at the
decay of the RIs and another type being a type of imaging using a positron
nuclide emitting a pair of photons in opposite directions at the decay of
a positron. Here, the former type is an ordinary one. Further, the method
of imaging includes the static imaging, the SPECT imaging, the two camera
head opposed SPECT imaging, the two camera head 90.degree.-displaced SPECT
imaging, three camera head SPECT imaging, and so on. Here, the static
imaging will be explained by way of example.
(First Embodiment)
FIG. 2 is a block diagram showing an arrangement of a gamma camera
according to a first embodiment of the present invention. A radio-isotope
(RIs) administered to a human subject emits a gamma ray (photon)
intermittently. A camera head 1 detects the photon and outputs an XY
signal representing its incident position and a Z signal representing an
energy of the photon. The camera head is, for example, two types, one
being an Anger type equipped with the existing photomultiplier (PMT) and
another type being a currently popular semiconductor array type. In the
present invention, either type may be used. The Anger type has a camera
head equipped with a collimator for restricting the incident direction of
the photon, a scintillator for producing scintillations of light by the
photons passed through the collimator, and a plurality of photomultipliers
for converting the scintillation light to an electric signal. The
semiconductor array type is such that, behind the collimator, a plurality
of semiconductor elements, such as CdTe and CdZnTe, are arranged in a
two-dimensional array to convert the photons to an electric signal.
The gamma camera of the present invention uses a TEW (triple energy window)
technique to effect scattering correction and crosstalk correction. The
TEW requires not-overlapped three energy windows.
The three energy windows comprise one main window and two sub-windows. The
main window is centered at a photoelectric peak inherent in a target
nuclide and two windows are arranged one adjacent the sub-window. It is to
be noted that the sub-window defined adjacent the main window at a low
side is referred to as a "low side sub-window" and the sub-window at a
high side as a "high side sub-window". The scattering and crosstalk
components mixed into the main windows are estimated, by a trapezium
approximation calculation, based on the number of photons passed through
the two sub-windows. The number of scattering components, etc., thus
estimated is subtracted from the number of the photons passed through the
main window. By doing so, it is possible to obtain the number of primary
photons with the scattering and crosstalk components corrected.
In order to realize the TEW in a hardwarewise, three window circuits 2A, 2B
and 2C are provided. First, the first window circuit 2A produces one pulse
when an energy representing a Z signal from the camera head 1 enters the
main window and the second window circuit 2B produces one pulse when an
energy representing a Z signal enters the low side sub-window. And the
third window circuit 2C produces one pulse when an energy representing a Z
signal enters the high side sub-window.
Image memories 4A, 4B and 4C correspond to the window circuits 2A, 2B and
2C, respectively. Upon receipt of one pulse from any of the window
circuits 2A, 2B and 2C, an image memory controller 3 delivers a control
signal to the corresponding image memories 4A, 4B and 4C, the control
signal incrementing a value of an address, by one, corresponding to the
incident position in the corresponding image memories 4A, 4B and 4C. This
action is continued for a predetermined time period. By doing so, the
number of photons passed through the main window is counted at each
incident position and stored in the image memory 4A. Similarly, the number
of photons passing through the low side sub-window is counted at each
incident position and stored in the image memory 4B. And the number of
photons passed through the high side sub-window is counted and stored in
the image memory 4C.
It is to be noted that data stored in the image memory 4A represents the
concentration distribution (planar image) of RIs releasing main
window-passed photons, referred to as "main image data" for convenience
sake, and, similarly, data stored in the image memories 4B and 4C as "low
side image data" and "high side image data", respectively.
Image processors 5A, 5B and 5C correct the image data stored in the image
memories 4A, 4B and 4C, respectively. The correction includes the energy
correction, the linear correction for correcting a distortion at the
marginal edge of a visual field, and the correction for uniforming a
variation in sensitivity of the photomultiplier and semiconductor device.
A scattering correction processor 6 performs the scattering correction of,
in accordance with the TEW technique, eliminating scattering components
which are mixed in the main image data on the basis of the low side image
data and high side image data. The display 10 displays
scattering-corrected main image data.
An operation panel 8 is provided for inputting the information relating to
the nuclide administered to the human subject to a system controller 9.
The system controller 9 transfers the information relating to the nuclide
administered to the human subject to the window controller 7. The window
controller 7 adjusts the energy windows (main window, low side sub-window
and high side sub-window) of the above-mentioned window circuits 2A, 2B
and 2C in accordance with the nuclide administered to the human subject.
The window controller 7 stores, associated with the nuclide information,
the information relating to the center energy and energy width of the
respective window necessary to the setting of the respective energy
windows of the window circuits 2A, 2B and 2C.
The present invention is directed to optimizing the center position and
width of the three energy windows in accordance with the nuclide. The
optimized three energy windows are determined as will be explained below.
As the RIs administered to the human subject, there are various nuclides
such as Tc-99m and TI-201. These nuclides are selected in accordance with
the diagnostic object. As well-known, the energy corresponding to the
maximal photon number, that is, the photoelectric peak, is inherent in the
nuclide. The energy is, for example, 140 keV for the Tc-99m and 71 keV for
the TI-201. The energy resolution is determined depending upon the energy
resolution inherent in the apparatus and photoelectric peak. The energy
resolution is determined as a difference of an energy in a pair
corresponding to the photon number of 21/2 times the maximum photon
number, that is, as the half width (FWHM). The photoelectric peak and
energy resolution are determined depending upon the nuclide.
The inventors have determined the optimal three energy windows in
accordance with the energy distribution (standard energy spectrum) of the
photon number relating to a direct ray free from any scattering ray, that
is, a ray directly incident to the camera head 1 from the RIs. First, the
inventors approximate the standard energy spectrum to the Gaussian
distribution (normal distribution) unconditionally determined from the
photoelectric peak "Epeak" and energy resolution "FWHM".
The high side sub-window is centered on an energy E.sub.H corresponding to
the photon number of 1/m times the maximum photon number "MAX"
corresponding to the photoelectric peak on the standard energy spectrum.
For the Anger type of camera head 1, the high side sub-window is
determined to be a width of 0.07 times the photoelectric peak "Epeak" and,
for the semiconductor type of camera head 1, to be a width of 0.03 times
the photoelectric peak "Epeak".
Further, the low side sub-window is centered on the energy E.sub.L
corresponding to the photon number of 1/n times the maximum photon number
"MAX" corresponding to the photoelectric peak on the standard energy
spectrum. Like the high side sub-window E.sub.L, the low side sub-window
is determined to be a width of 0.07 times of the photoelectric peak
"Epeak" for the Anger type of camera head 1 and to be a width of 0.03
times of the photoelectric peak "Epeak" for the semiconductor type of
camera head 1.
The main window is determined between the low side sub-window and the high
side sub-window. That is, the main window is determined in a range from
the maximum energy of the low side sub-window to the minimum energy of the
high side sub-window.
The "m" and "n" represent a rational number exceeding 1. In the experiment
conducted by the inventors, m=n=100 at which the most optimal window is
designed. It is to be noted that, if m=n=100.+-.10% (90-100), the
optimization falls in an allowable range.
The number "m" may be adjusted in accordance with the energy (scattering
peak) at which the photon number of the scattering ray is maximal. The
scattering peak is associated with the photoelectric peak and can be
measured with the use of a phantom. As shown in FIG. 4, an amount of
scattering ray mixed into the main window can be reduced by adjusting the
"m" so as to have the center energy of the low side sub-window set at a
position between the photoelectric peak E.sub.p-peak and the scattering
peak E.sub.s-peak.
For the Tc-99m, for example, the photoelectric peak is 140 keV and the
energy resolution is 16.5 keV. If, in this case, the width of the
sub-window is set to 7% of the photoelectric peak, the low side sub-window
is set to 116.2 keV to 126.0 keV, the high side sub-window is set to 154.0
keV to 163.8 keV and the main window is set to 126.0 keV to 154.0 keV. At
this time, the energy width of the main window becomes about 20.0% of the
photoelectric peak.
For the TI-201, the photoelectric peak is 74 keV and the energy resolution
is 12.6 keV. Even in this case, if the width of the sub-window is set to
7% of the photoelectric peak, the low side sub-window is 51.3 keV to 56.5
keV, the high side sub-window is 91.5 keV to 96.7 keV and the main window
is 56.5 keV to 91.5 keV. And the energy width becomes about 47.3% of the
photoelectric peak.
If, in this example, the center of the low side sub-window becomes 53.9 keV
and that of the high side sub-window becomes 94.1 keV, then it is
possible, according to the present invention, to obtain substantially the
same effect if the center of the low side sub-window is located in a range
from 51 keV to 56 keV and the center of the high side window is located in
a range from 92 keV to 97 keV.
The main window and sub-window determined in accordance with the
photoelectric peak of the nuclide and energy window have their scattering
components not underestimated. And it is possible to improve the counting
efficiency, by making the main window as wide as possible, and to optimize
the energy window.
(Second Embodiment)
FIG. 5 shows an arrangement of a gamma camera according to a second
embodiment. In FIG. 5, the same reference numerals are employed to
designate parts or elements corresponding to those shown in FIG. 2 and any
further explanation is, therefore, omitted. The gamma camera of this
embodiment operates in two operation modes. The first mode is for an
operation for creating an absorption coefficient map relation to an
external ray source. The second mode is a mode effected subsequent to the
first mode and is for an operation for subjecting the number of photons
which come from RIs administered to the human subject to absorption
correction with the use to an absorption coefficient map created at the
first mode and for obtaining the concentration distribution (planar image,
SPECT image, PET image) from a result of correction.
The optimization of the window of the first embodiment can also be applied
to the creation of an absorption coefficient map in the first mode of the
present embodiment. In the first mode, a plane-like external ray source 12
whose photon radiation rate is spatially uniform is arranged in a position
opposite to the camera head 1 with the human subject therebetween. The
photons radiated from the external ray source 12 and passed through the
human subject 12 are counted for a predetermined period through a camera
head 1. The number of photons radiated from the external ray source 12
during this period is already known and, with this number given by
"I.sub.0 ", the following relation is established:
I.sub.1 =I.sub.0.multidot.e.sup.-.mu..multidot.d
where
I.sub.1 : the number of photons passed through the human subject;
.mu.: the absorption efficient; and
d: the thickness of the human subject.
From this relation it is possible to find the absorption coefficient
".mu.", by an absorption coefficient map processor 11, from this relation.
In order to improve the precision of the absorption coefficient, it is
effective to eliminate scattering components, with high precision, by the
TEW technique. For the first mode, the data held in the image memory 4A
represents a spatial distribution corresponding to the number of photons
radiated from the ray source, passed through the human subject and passed
through the main window, and is referred to as main transmission image
data for convenience sake. The data retained in the image memories 4B and
4C are referred to, similarly, as the row side transmission image data and
high side transmission image data, respectively.
In accordance with the TEW technique, the collected main transmission image
data is corrected by scatter coefficient processor 6 on the basis of the
low side transmission image data and high side transmission image data.
The absorption coefficient map is created under the absorption coefficient
map processor 11 on the basis of the amended main transmission image data.
In the case where the absorption coefficient map is created with the use of
such external ray source 12, it is possible to, by optimizing the three
windows in accordance with the nuclide as set out above, improve the
counting efficiency while broadening the main window to a maximal possible
extent and do this without underestimating any scattering component. By
doing so it is possible to generate the absorption coefficient map with
high precision.
(Third Embodiment)
FIG. 6 shows an arrangement of a major section of a gamma camera according
to a third embodiment of this invention. For the first and second
embodiments, the TEW technique is realized in the hardware fashion by
providing the window circuits 2A, 2B, 2C, etc., while, for the third
embodiment, it is realizing, in a software function, under a
computer-readable/computer-executable program stored in a storage medium,
such as a magnetic disk.
An energy spectrum collection unit 21 collects an energy spectrum at each
incident position on the basis of an output of the camera head (camera
body). The energy spectrum is obtained by collecting count values on a
plurality of very small windows of narrow width continuously arranged at
different energy values. A processor 23 executes a program code. As in the
first embodiment, the program code has means for determining the widths
and positions of a main window, low side window, high side window in
accordance with the kind of input nuclide, means for counting the number
of photons corresponding to the main window, low side window and high side
window by adding together collected count values of the very small windows
in the energy spectrum, and means for making the calculation of
eliminating scattering rays by the TEW technique from the number of
photons counted.
In this way, the present invention can also be applied to the case of
implementing the TEW technique with the use of the program code.
The present invention is not restricted to the above-mentioned embodiments
and various changes or modifications of the present invention can be made
without departing from the spirit and scope of the present invention.
Additional advantages and modifications will readily occur to those skilled
in the art. Therefore, the invention in its broader aspects is not limited
to the specific details and representative embodiments shown and described
herein. Accordingly, various modifications may be made without departing
from the spirit or scope of the general inventive concept as defined by
the appended claims and their equivalents.
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